katex [nfc]: Rename a local to hasVlistT2

This reflects the condition that's directly encoded by this boolean.
(Through several conditionals in the rest of the function, we end up
confirming that this value is true just if the element does in fact
have two `.vlist-r` children, as the old name suggests.  But that
comes later and more indirectly.)

Author: gnprice

lib/model/katex.dart

@@ -258,10 +258,9 @@ class _KatexParser {
     if (vlistT.nodes.isEmpty) throw _KatexHtmlParseError();
     if (vlistT.attributes.containsKey('style')) throw _KatexHtmlParseError();
 
-    final hasTwoVlistR = vlistT.className == 'vlist-t vlist-t2';
-    if (!hasTwoVlistR && vlistT.nodes.length != 1) throw _KatexHtmlParseError();
-
-    if (hasTwoVlistR) {
+    final hasVlistT2 = vlistT.className == 'vlist-t vlist-t2';
+    if (!hasVlistT2 && vlistT.nodes.length != 1) throw _KatexHtmlParseError();
+    if (hasVlistT2) {
       if (vlistT.nodes case [
         _,
         dom.Element(localName: 'span', className: 'vlist-r', nodes: [